Dogs
The American Heartworm Society recommends regular testing and consistent prevention; treatment requires veterinary staging and supervision.
INTERNAL PARASITE OWNER GUIDE
Dirofilaria immitis
Mosquito-transmitted worms that primarily affect the pulmonary arteries and can cause severe heart and lung disease in dogs, cats and ferrets.
THE IMPORTANT DIFFERENCE
Many parasites produce similar signs, and some are microscopic or hidden inside the body. A veterinarian can choose the right examination, testing and species-safe prevention plan.

Adult heartworms are long, thin, pale worms inside the cardiopulmonary circulation and cannot be identified by looking at an animal. Immature circulating stages, called microfilariae, are microscopic.
Heartworms are transmitted through mosquitoes, and prevention, testing, and treatment have different roles. A prevention product does not replace the tests appropriate for the species and exposure history. Dogs, cats, and ferrets also differ in disease patterns and treatment options. A positive result needs veterinary interpretation and a species-specific plan. For a dog undergoing treatment, prescribed activity restriction is important because the complications involve the circulation and lungs, not only the presence of worms.
The exact parasite species, exposure route and appropriate treatment may differ between these animals.
The American Heartworm Society recommends regular testing and consistent prevention; treatment requires veterinary staging and supervision.
Even a small number of worms or immature stages can cause serious respiratory disease, and the canine adulticide treatment is not used in cats.
Ferrets can be susceptible and should have an individualized prevention discussion with an experienced veterinarian.
Animals can have a parasite without showing every listed sign. Difficulty breathing, collapse, severe weakness, pale gums or rapidly worsening illness require urgent veterinary attention.
Blood-cell testing and a chemistry profile provide complementary information. Cell counts can help assess anemia, inflammation, and platelets; chemistry measurements help assess organs and internal balance. Results are interpreted against the species, examination, hydration, medication history, and previous values. A value just outside the reference interval is not automatically the cause of illness. Trends can be more useful than one result, particularly when the aim is to monitor a chronic problem or check that treatment remains safe.
Diagnostic choices depend on the suspected parasite, stage of infection, animal species and clinical history.
Parasite control depends on the organism, its life cycle, and the host species. A medicine effective against one parasite may not cover another, and a product safe for one species may be dangerous for a different animal. Testing may be needed before treatment and again when assessing response or ongoing exposure. Environmental stages and untreated contacts can contribute to reinfection. The plan therefore may combine a specific medicine with cleaning, husbandry, and prevention instead of relying on a single dose.
A preventive plan depends on the product's labeled species, age, weight, target organisms, route, and administration interval. Similar names or packaging do not establish that products are interchangeable. Some cover parasites that another does not. Review the current label and patient history with the veterinarian, including previous reactions and other medicines. Record actual administration rather than purchase dates. If a dose is missed or given incorrectly, obtain product-specific advice instead of doubling, combining, or substituting a treatment independently.
Risk can involve contact animals, vectors, travel, boarding, new arrivals, equipment, food, water, or wildlife, depending on the organism. Indoor housing or a fenced property may reduce some exposures without eliminating all of them. Conversely, prevention should still be tailored rather than chosen without regard to the actual setting. Describe the household or group accurately and review changes over time. Geographic summaries and maps are useful context, but cannot establish an individual animal's infection status or exact risk by themselves.
People do not catch heartworm by touching or living with an infected pet. Rare human infection can occur through an infected mosquito, so the meaningful connection is shared vector exposure rather than direct pet-to-person spread.
Speak with a physician about personal medical concerns, especially during pregnancy, immune suppression or significant exposure. Ask your veterinarian what precautions are appropriate for the animal and household.
This educational guide does not replace veterinary examination, diagnostic testing, individualized treatment or human medical advice.